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Top Physics at the Tevatron

Top Physics at the Tevatron. David Gerdes University of Michigan representing the CDF and DØ Collaborations IInd International Conference on Flavor Physics (ICFP2003) KIAS, Seoul October 9, 2003. Motivations for Studying Top. Only known fermion with a mass at the natural electroweak scale

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Top Physics at the Tevatron

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  1. Top Physics at the Tevatron David Gerdes University of Michigan representing the CDF and DØ Collaborations IInd International Conference on Flavor Physics (ICFP2003) KIAS, Seoul October 9, 2003 David Gerdes University of Michigan

  2. Motivations for Studying Top • Only known fermion with a mass at the natural electroweak scale • Special role in precision electroweak physics • Window into the problem of EWSB? • New physics may appear in production (e.g. topcolor) or in decay (e.g. charged Higgs). • Can only be studied at Tevatron prior to LHC. David Gerdes University of Michigan

  3. A Brief History of Top • Observed in 1995 in first ~70 pb-1 of Run I data. • Final Run I top analyses based on ~110 pb-1. • Production cross sections in many channels • Mass: 174.3  5.1 GeV (CDF/DØ combined) • Event kinematics • W helicity measurement • Limits on single top production, rare/non-SM decays • Overall consistency with the Standard Model. • But only ~100 analyzable top events →analyses statistics-limited. David Gerdes University of Michigan

  4. Improvements for Run II • Accelerator • Energy upgrade: 1.8 → 1.96 TeV • 30-40% increase in top cross section • Luminosity upgrades: factor of ~2-3 so far • Detectors • CDF: new Si vertex detector, outer tracker, endplug calorimeter, extended muon coverage • DØ: magnetic tracking system (scint. fibers + silicon), preshower system, muon upgrades • Both: Upgraded DAQ/trigger systems to deal with change from 3.5μs to 396 ns bunch crossing interval. David Gerdes University of Michigan

  5. Tevatron Peak Luminosity Typical recent stores: 3-4x1031 Run IIa goal: 8x1031 David Gerdes University of Michigan

  6. Integrated Luminosity delivered ~325 pb-1 on tape ~270 pb-1 Start of physics-quality data Taking data with ~90% efficiency Results from first ~100 pb-1 presented today. Goal for 2004: additional 310-380 pb-1 delivered. David Gerdes University of Michigan

  7. Production and Decay Basics Pair Production: b 85% Event topology determined by the decay modes of the W’s t W 15% t W σtheory ≈ 7 pb b NB: qq, gg fractions reversed at LHC b-jet: identify via secondary vertex or soft lepton tag BR( t → Wb) ≈ 100% David Gerdes University of Michigan

  8. Dilepton (ee, μμ, eμ) BR = 5% 2 high-PT leptons + 2 b-jets + missing-ET Lepton (e or μ) + jets BR = 30% single lepton + 4 jets (2 from b’s) + missing-ET All-hadronic BR = 44% six jets, no missing-ET τhad+X BR = 23% t-tbar Final States Most favorable channels for top physics More challenging backgrounds, but measurements still possible David Gerdes University of Michigan

  9. Measuring the ttbar Cross Section • Basic engineering number, starting point for all top physics. • Requires detailed understanding of backgrounds and selection efficiencies. • Test of QCD • Latest calculations: NNLO + NNNLL • Departures from prediction could indicate nonstandard production mechanisms, i.e. production through decays of SUSY states. David Gerdes University of Michigan

  10. Dilepton Cross Sections: DØ Results from first 90 – 110 pb-1 • ee channel • Observe 2 events, bkgd. 0.6 ± 0.5 • μμ channel • Observe 0 events, bkgd. 0.7 ± 0.4 • eμ channel • Observe 3 events, bkgd. 0.4 ± 0.4) David Gerdes University of Michigan

  11. Dilepton Cross Section: CDF Two complementary analyses (126 pb-1) • Tight: Two good-quality leptons + MET + 2 jets • 10 candidates (2 ee, 3 μμ, 5 eμ), bkgd. 2.9 ± 0.9 • 6 events b-tagged (one double-tag); expect 4 top • σtt = 7.6 ± 3.4 (stat) ± 1.5 (sys) pb • Loose: Lepton + isolated track + MET + 2 jets • 13 candidates, bkgd. 5.1 ± 0.9 • 5 events b-tagged (one double-tag); expect 4 top • σtt = 7.3 ± 3.4 (stat) ± 1.7 (sys) pb David Gerdes University of Michigan

  12. Jet Multiplicity in Dilepton Events ttbar signal bin David Gerdes University of Michigan

  13. Double b-tagged Lep+Trk event at CDF David Gerdes University of Michigan

  14. Dilepton Kinematics Scalar summed ET of jets, leptons, and missing ET David Gerdes University of Michigan

  15. Dilepton Kinematics, contd. Lepton PT softer than expected. Statistical fluctuation or a hint of something new? David Gerdes University of Michigan

  16. Lepton + Jets Cross Section: DØ Backgrounds from QCD estimated from data as fcn. of MET, Njets. Backgrounds from W+jetsestimated using Berends scaling hypothesis, σ(W+n+1 jets)/σ(W+n jets) = constant. After aplanarity, HT , Njet ≥ 4 cuts: observe 26 events, bkgd. 18.5 ± 2.5. Using topological cuts ℓ± ν Using soft muon b-tag Orthogonal selection to topological analysis. QCD and W+jets backgrounds estimated as in topological analysis. Fake tag rate estimated using jet data. ν ℓ± μ Observe 15 events, bkgd. 3.3 ± 1.3. David Gerdes University of Michigan

  17. Lepton + Jets Kinematics DØ Run II Preliminary μ+jets 94 pb-1 e+jets 92 pb-1 Aplanarity Aplanarity Sum-ET of jets (GeV) Sum-ET of jets (GeV) b-tagged events populate the top signal region David Gerdes University of Michigan

  18. Lepton + jets with Secondary Vertex B-Tag at DØ 45 pb-1 Tag by reconstructing sec. vtx.: Tag by counting displaced tracks: David Gerdes University of Michigan

  19. m - SV IP MTC Jet 2 Jet 1 IP SV Jet 1 m+jets double tagged event at DØ David Gerdes University of Michigan

  20. Jet Multiplicity in b-tagged events: CDF top signal region David Gerdes University of Michigan

  21. Jet Multiplicity (with top contribution) σl+jets = 4.5 ± 1.4(stat) ± 0.8(syst) pb New David Gerdes University of Michigan for ICFP2003

  22. L+jets: Tagged Jet ET David Gerdes University of Michigan

  23. Summary of Cross Section Results David Gerdes University of Michigan

  24. Cross Section √s-Dependence David Gerdes University of Michigan

  25. Top Mass Measurement Mtop is a precision electroweak parameter that helps constrain the mass of the Higgs. David Gerdes University of Michigan LEP EWWG summer ‘03

  26. Top Mass in Run II (CDF) • Lepton + 4 jets with sec. vertex b-tag • Many kinematic constraints: 4C fit • 12 parton/jet matching assignments possible; pick combination with lowest χ2. • Fit resulting to mass distribution to background + signal templates. • Dilepton channel • Underconstrained system • Use Pttbar,z to weight the mass fit distribution • Likelihood fit to top mass templates. David Gerdes University of Michigan

  27. Run II Top Mass: lepton + jets David Gerdes University of Michigan

  28. Run II Top Mass: Dilepton Channel 6 events 124 pb-1 David Gerdes University of Michigan

  29. New Run I Mass Measurement (DØ) • The template method has some disadvantages: • One combination chosen for fit • Single template describes the distribution • All events treated with equal weight • New analysis makes better use of available information • All measured quantities used in fit (except unclustered energy) • Each event has its own probability distribution • Well-measured events contribute more David Gerdes University of Michigan

  30. Matrix Element Method W(y,x) is the probability that a parton level set of variables y will be measured as a set of variables x dnσ is the differential cross section f(q) is the probability distribution than a parton will have a momentum q • Leading-Order ttbar->lepton+jets matrix element, PDFs • 12 jet permutations, all values of P() • Phase space of 6-object final state • Detector resolutions • Only W+jets, 80% • VECBOS subroutines for W+jets • Same detector resolutions as for signal • All permutations, all values of P() • Integration done over the jet energies • Convolute probability to include all conditions for accepting or rejecting an event • Form a Likelihood as a function of: Top Mass, F0(longitudinal fraction of W bosons) David Gerdes University of Michigan

  31. Error Comparable to Previous Run I Measurements Combined DØ Run I Preliminary Mtop = 180.1 ± 3.6 (stat) ± 4.0 (syst) GeV/c2 Previous DØ result using template method had stat. uncertainty of 5.6 GeV. New method is equivalent to 2.4 times more data! David Gerdes University of Michigan

  32. W Helicity Measurement • Top decays before it can hadronize, because width Γt = 1.4 GeV > ΛQCD. • Decay products preserve information about the underlying Lagrangian. • Unique opportunity to study the weak interactions of a bare quark, with a mass at the natural electroweak scale! • SM Prediction: • W helicity in top decays is fixed by Mtop, MW, and V-A structure of the tWb vertex. David Gerdes University of Michigan

  33. parameter to measure W Helicity Measurement, contd. The angular dependence of the semileptonic decay in the W rest frame is given by right left long. SM predictions (for mb=0): where= MW2/Mtop2 David Gerdes University of Michigan

  34. W Helicity Results • New DØ Run I measurement: • Natural extention of the ME method developed for top mass measurement. • Extend the ME to include generalized dependence on F0. F0= 0.56  0.31 (stat)0.04 (syst) L/Lmax • Application to Run II data is in progress David Gerdes University of Michigan

  35. Long+l.h.+bg Long Left-handed Background Data 108÷126 pb-1 Helicity affects lepton PT in lab frame See general agreement with SM, but limited statistics. Analysis in progress. David Gerdes University of Michigan

  36. Test for new physics in tt production Model independent search for a narrow resonance Xtt exclude a narrow, leptophobic X boson with mX < 560 GeV/c2 (CDF) and mX < 585 GeV/c2 (D0) David Gerdes University of Michigan

  37. Tevatron Luminosity Projections With recycler and electron cooling David Gerdes University of Michigan

  38. Conclusions and Outlook • The top quark is back! • First Run II measurements of cross section, mass are available and will improve rapidly. • Other analyses (W helicity, single top…) are making excellent progress. • It is the start of a program of precision top physics—and hopefully top surprises—at the Tevatron. • We still expect at least 50x more data compared to Run I! David Gerdes University of Michigan

  39. The Road Ahead • Search for top  H+ • Study of  channels – pure 3rd generation decay mode. • Single top production, measure Vtb • ttbar resonant production, strong EWSB • Searches for rare decays • Is top*the connection*to new physics? David Gerdes University of Michigan

  40. Top Mass Uncertainties, lepton + jets CDF Run II Preliminary Dominated by calorimeter energy scale in simulation; will improve soon David Gerdes University of Michigan

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